The present invention relates to a method as defined in the preamble. of claim 1 for controlling a permanent magnet synchronous motor and to a control loop as defined in the preamble of claim 6.
A conventional method in the control of an alternating-current motor is to convert from stator co-ordinates to rotor co-ordinates. and back to stator co-ordinates. This conversion serves to simplify the analysis of the alternating-current motor; in the rotor co-ordinates, the control quantities are interpreted at the direct. current level. In a practical implementation, effecting the conversions requires extra resources either in software or in electronics. The motor torque is generated as a vector product of magnetic flux and current, which means that the current and voltage have to be either as precisely as possible in phase or exactly in opposite .phase. Because of non-ideal properties, such as motor inductance or stator resistance, the system oscillates about the point of balance, with the result that passengers travelling in the elevator experience an unpleasant sensation. Typically, the oscillation frequency is low, about 2 . . . 5 Hz.
The object of the present invention is to eliminate the problems described above. A specific object of the present invention is to. disclose a new type of method for the control of a synchronous motor. The method simplifies the motor control, allowing a more advantageous implementation to be achieved than before. Another object is to achieve a control system that is particularly well suited for a permanent magnet synchronous motor used to drive an elevator.
As for the features characteristic of the present invention, reference is made to the claims.
In the method of the invention, an equivalent circuit describing the properties of a permanent magnet. synchronous motor is formed. Via calculations based on the equivalent circuit, a vectorial representation. of the control quantities is produced, in which the horizontal axis of the co-ordinate system used represents the magnetisation, the vertical axis represents the torque and the vectors used are the stator voltage, the supply voltage and the current, which is at a distance of 90xc2x0 from the horizontal axis. Therefore, the angle of the current may be +90xc2x0 or xe2x88x9290xc2x0. In the vectorial representation, a correction vector is produced via inference preferably based on the equivalent circuit. The correction vector is summed with the electromotive force, and the result produced is the stator voltage or a voltage reference for the stator voltage required. The method makes it possible to advantageously reduce the amplification factor needed in the current feedback loop. For example, the amplification factor required without correction is 10 . . . 30, whereas the amplification factor needed after correction is 1 . . . 5. A lower amplification allows several advantages to be achieved; for instance, the system""s sensitivity to interference is reduced.
In a preferred embodiment of the present invention, the motor voltage is calculated by means of an analogue calculator using an analogue electromotive force estimator and current feedback.
In a preferred embodiment of the present invention, each motor phase is controlled separately.
In a preferred embodiment of the present invention, the calculation of the correction vector is dependent on the torque required.
In a preferred embodiment of the present invention, the calculation of the correction vector is implemented using an operational amplifier circuit.
When a permanent magnet synchronous motor driving an elevator is controlled using the method and/or control loop of the invention, the speed of regulation allows a better controllability of elevator motion and an improved travelling comfort to be achieved. Especially processing the signals involved in the regulation in analogue form is a factor that permits fast feedback in the correction of any deviations.